The present invention relates to an isolating circuit and a pair of dielectric filters for use therein. More particularly, the invention relates to an isolating circuit, such as a duplexer or an isolator, for isolating a first frequency signal in a first frequency range and a second frequency signal in a second frequency range which is higher than the first frequency range, and still more particularly a duplexer well adapted for a mobile telephone.
The demand for mobile telephone services in large cities such as New York, London, Tokyo etc. has suddenly expanded more than initially expected. This has caused a shortage in the number of channels for communication services. In order to solve this shortage, there are mobile telephone service corporations which are planning or which have already carried out an increase in the number of channels, for example, from 666 channels to 832 channels in the U.S. and from 600 channels to 1320 channels in the U.K., as described in a publication by Tomokazu Komazaki et al. entitled "Dielectric Filter with Attenuation Pole for Mobile Radio", Electron Information Communication Society, CAS88-10, dated June 23, 1988, the entire disclosure of which is incorporated by reference.
In accordance with the channel number increase, it is necessary to extend the bandwidth for radio communications. In the U.S., the transmitting frequency band and receiving frequency band have respectively extended from 825-845 MHz to 824-849 MHz and from 870-890 MHz to 869 894 MHz. As a consequence, a duplexer is required so as to more effectively isolate the transmitter and the receiver to permit simultaneous operation since the transmitting and receiving frequencies are more closely spaced. Dielectric filters which may be used in such a duplexer are disclosed in Japanese laid-open patent publication Nos. 62-77703 (published on Apr. 9, 1987) and 62-157402 (published on July 13, 1987).
A dielectric filter, disclosed in Japanese laid-open patent publication No. 62-77703 has six dielectric resonators and a reactance circuit formed by a capacitor or an inductor. The reactance circuit, jumping over at least one resonator, connects two resonators out of the remaining resonators of the dielectric filter. As a result the dielectric filters each have an attenuation pole.
Dielectric filter disclosed in Japanese laid-open patent publication No. 62-157402 has four dielectric resonators therein and a coaxial cable having two edge portions. The coaxial cable, jumping over two resonators, couples the two remaining resonators of the dielectric filter through two reactance components, respectively connected to two edge portions thereof. As a result the dielectric filters have two attenuation poles which are asymmetric relative to the center frequency.